The present invention relates to a desulfurizing process and apparatus by irradiation of electron beam, and more particularly to a process and an apparatus for desulfurization by injection of ammonia into a high temperature gas containing sulfur oxides such as a combustion flue gas discharged from a boiler and irradiation of electron beam.
As economy develops, more and more energy is demanded. Amidst the continuous growth of energy demand, energy source is still dependent on fossil fuels such as coal and petroleum. However, the harmful products or pollutants generated by the burning of fossil fuels are responsible for global pollution. To prevent the release of pollutants into the atmosphere and to stop the pollution of global environment, development work is being carried out at an accelerated pace to create a flue gas treatment system for installation in fuel combustion plant such as thermal power plants. There are still many areas of improvement to meet problems such as the complicated configuration of the equipment requiring a large number of control variables and the need for large-scale waste water treatment systems requiring sophisticated treatment technology.
In an effort to solve these problems, a flue gas treatment system in which flue gas discharged from the fuel combustion facility such as a boiler is treated by irradiation of electron beam has been developed.
In this system, ammonia is injected into a high temperature gas containing sulfur oxides, and the mixed gas is irradiated with electron beam to remove the sulfur oxides therefrom in the form of a powder of ammonium compounds. In this case, the lower the temperature of gas is, the higher the reaction rate between sulfur oxides and ammonia is. Therefore, the high temperature gas is required to be cooled to a certain range of temperature. Accordingly, conventionally, the gas is normally cooled to a temperature ranging from an adiabatic saturation temperature plus 10xc2x0 C. to 80xc2x0 C. by water spray at the top of a cooling tower which is located in the system upstream of a process vessel in which ammonia is injected. The cooling tower is a complete evaporating type in which the sprayed water is completely evaporated. The gas cooling system has such an advantage that no waste water treatment system is required to be installed because of generation of no waste water. Further, by controlling the amount of sprayed water, the temperature of gas discharged from the cooling tower can be adjusted.
However, if the concentration of sulfur oxides is relatively large and/or the dose of electron beam is relatively large, an increase of the gas temperature caused by the heat of reaction between sulfur oxides and ammonia and/or by the heat generation due to irradiation of electron beam is not negligible. Therefore, in order to avoid lowering the reaction rate due to an increase of the gas temperature, after cooling of the gas in the cooling tower, it is necessary to adjust the gas temperature in the range of 50 to 80xc2x0 C. by water spray in a process vessel. At this time, the sprayed water is completely evaporated in the process vessel or in the subsequent stage, and hence waste water is not generated in the process vessel as well (hereinafter suppression of an increase of the gas temperature by evaporation of sprayed water in the process vessel is referred to as xe2x80x9csecondary gas coolingxe2x80x9d, and the cooling of the gas before the reaction step is referred to as xe2x80x9cprimary gas coolingxe2x80x9d).
FIG. 3 is a schematic view of a conventional electron beam flue gas treatment system. As shown in FIG. 3, flue gas containing sulfur oxides discharged from a boiler 1 which is a kind of fuel combustion facility is cooled in a heat exchanger 2, and then introduced into a cooling tower 4. In the cooling tower 4, water supplied from a pump 3 is sprayed by a single-fluid nozzle 6, and the sprayed water is completely evaporated therein. The cooling tower 4 is a complete evaporating type in which the sprayed water is completely evaporated. The flue gas is cooled to a certain range of temperature in the cooling tower 4, and then the cooled gas is introduced into a process vessel 5.
On the other hand, ammonia supplied from an ammonia supply equipment 9 is mixed with air in a line mixer 10. The mixed gas and water supplied from a water supply source (not shown) are mixed in a gas-liquid mixing room of a two-fluid nozzle 11, and sprayed at the entrance of the process vessel 5. The mixture of the gas and water are irradiated with electron beam from an electron accelerator 12.
According to the conventional method in which the primary gas cooling is conducted in a complete evaporating type cooling tower, a gas retention time of 10 to 30 seconds in the cooling tower is required to evaporate the sprayed water completely in the cooling tower. This causes the problems that a large volume cooling tower is needed, and a high construction cost of the cooling tower and a large space for the cooling tower are required. It is possible to decrease the volume of the cooling tower by decreasing the diameter of the droplets of the sprayed water. However, this method is problematic in that more power is required for atomization of the water into fine droplets.
It is therefore an object of the present invention to solve the above problems, and to provide a desulfurizing process and apparatus which can reduce greatly a gas retention time in a cooling tower, a volume of the cooling tower, a construction cost of the cooling tower and an installation space for the cooling tower without increasing the required power, while maintaining advantages of being free from waste water in the conventional method.
In order to achieve the above object, the inventors of the present application proposes a method in which the primary gas cooling is conducted by contacting the gas with recirculating cooling water (hereinafter referred to as xe2x80x9cwater recirculating cooling methodxe2x80x9d). In this case, the gas retention time in the cooling tower is reduced significantly to 0.1 to 5 seconds, and hence the volume of the cooling tower may be extremely small, and the construction cost and the installation space of the cooling tower may be significantly decreased. Further, since there is no need for atomization of the water, the required power can be lessened. However, since sulfur oxides, soot or dust contained in the gas is captured in the recirculating cooling water, it is necessary to partially withdraw water from the recirculating cooling water to stabilize cooling of gas. The withdrawn water, which is waste water, is discharged to the environment after necessary treatment. This fails to maintain the advantages that the process is free from waste water in the conventional technique. Therefore, the inventors of the present application have investigated further, and have invented a novel method in which the water withdrawn partially from the recirculating cooling water is used for the secondary gas cooling.
According to one aspect of the present invention, there is provided a desulfurizing process in which a high temperature gas containing sulfur oxides is processed to remove sulfur oxides by converting the sulfur oxides into ammonium compounds with injection of ammonia and irradiation of electron beam, the process comprising the steps of: cooling the high temperature gas by being contacted with recirculating cooling water to obtain a cooled gas; withdrawing a portion of the recirculating cooling water; spraying the withdrawn water or the withdrawn water that is diluted with water into the cooled gas before, or simultaneously with, or after injection of ammonia, or by mixing with ammonia into the cooled gas; and evaporating the sprayed water completely.
With the above process, an increase of the gas temperature caused by the heat of reaction between sulfur oxides and ammonia and/or by the heat generation due to irradiation of electron beam is suppressed, and the gas temperature at the outlet of the process vessel is adjusted in the range of 50 to 80xc2x0 C. so that no waste water is generated. It is preferable to filter the water withdrawn partially from the recirculating cooling water before it is sprayed in the process vessel.
Further, in the conventional technique, the cooling tower is required to be extremely large in volume to cool the gas to an adiabatic saturation temperature plus 10xc2x0 C. or less. In contrast thereto, in the present invention, by controlling the recirculating flow rate of the recirculating cooling water, the gas temperature at the outlet of the cooling tower can be adjusted in the range from the adiabatic saturation temperature to 80xc2x0 C. suitable for reaction.
The amount of sprayed water required for the secondary gas cooling depends on the gas temperature before the secondary gas cooling, the gas temperature to be achieved by the secondary gas cooling, the heat of reaction, and the dose of electron beam. In order not to generate waste water, it is necessary that the amount of water withdrawn partially from the recirculating cooling water is equal to or less than the amount of sprayed water required for the secondary gas cooling. In the case where sulfur oxide concentration is 100 ppm or more and/or the dose of electron beam is 2 kGy or more, the amount of sprayed water required for the secondary gas cooling becomes large, and the amount of sprayed water required for the secondary gas cooling is sufficiently larger than that of the withdrawn water from the recirculating cooling water, and hence the present invention is particularly suitable for such cases.
According to another aspect of the present invention, there is provided a desulfurizing apparatus in which ammonia is injected into a high temperature gas containing sulfur oxides and mixed gas is irradiated with electron beam to remove sulfur oxides in the form of a powder of ammonium compounds, the apparatus comprising: a gas cooling device for cooling the high temperature gas by being contacted with recirculating cooling water to obtain a cooled gas; an ammonia injecting device for injecting ammonia into the cooled gas; and a spraying device located upstream of, at the same position as, or downstream of the ammonia injecting device, for spraying water withdrawn from the recirculating cooling water, or withdrawn water that is diluted with water into the cooled gas.
According to still another aspect of the present invention, there is provided a desulfurizing apparatus in which ammonia is injected into a high temperature gas containing sulfur oxides and mixed gas is irradiated with electron beam to remove sulfur oxides in the form of a powder of ammonium compounds, the apparatus comprising: a gas cooling device for cooling the high temperature gas by being contacted with recirculating cooling water to obtain a cooled gas; a mixing device for mixing ammonia with water withdrawn from the recirculating cooling water, or with withdrawn water that is diluted with water; and a spraying device for spraying a mixture of ammonia and the withdrawn water or a mixture of ammonia and diluted withdrawn water into the cooled gas.
According to still another aspect of the present invention, there is provided a desulfurizing apparatus in which ammonia is injected into a high temperature gas containing sulfur oxides and mixed gas is irradiated with electron beam to remove sulfur oxides in the form of a powder of ammonium compounds, the apparatus comprising: a gas cooling device for cooling the high temperature gas by being contacted with recirculating cooling water to obtain a cooled gas; and a gas-liquid mixture spraying device for mixing ammonia or a mixture of ammonia and air, with water withdrawn from the recirculating cooling water or withdrawn water that is diluted with water and spraying a gas-liquid mixture produced by the mixing into the cooled gas.
In the above apparatus, it is preferable to provide a filtering device for filtering the water withdrawn partially from the recirculating cooling water in the following stage of the gas cooling device.
In the present invention, the amount of water withdrawn partially from the recirculating cooling water is adjusted so that pH of the recirculating cooling water is in the range of 1 to 7, preferably 1 to 6. Alternatively, pH of the recirculating cooling water is adjusted to be in the range of 1 to 7, preferably 1 to 6 by the addition of at least one of alkaline compounds selected from the group consisting of ammonia, sodium compounds, potassium compounds, calcium compounds and magnesium compounds. The water withdrawn partially from the recirculating cooling water is adjusted to have pH of from 1 to 8, preferably from 1 to 7, by the addition of at least one of alkaline compounds selected from the group consisting of ammonia, sodium compounds, potassium compounds, calcium compounds and magnesium compounds, or by dilution with water. Soft water may be used as make-up water for the recirculating cooling water of the gas cooling device, i.e., the cooling tower. A hydroxide, oxide or carbonate of sodium, potassium, calcium, magnesium and the like can be used as the alkaline compounds.